As demands for hybrid vehicles, pure electric vehicles and energy-efficient air-conditioning compressor are growing, demands for rare earth permanent magnet material (such as an R—Fe—B-based rare earth permanent magnet) with a high coercive force are growing. Conventional methods for increasing coercive force need to use a large amount of heavy rare earth element, resulting in a significant increase in cost of magnets and a sacrifice of parts of remanence and energy product. Microscopic studies have showed that the grain boundary plays an important role in increasing the coercive force of magnets. The heavy rare earth element goes into grain boundaries by diffusion and infiltration (referred to as infiltration), so that the coercive force can be significantly increased by using less heavy rare earth, without sacrificing the remanence and magnetic energy product, which effectively reduces the cost of magnets.
There have been some methods in the prior art which improve grain boundaries by diffusion and infiltration. However, an increase of coercive force normally bring adverse effects such as a significant decrease of remanence and magnetic energy product, a large amount of heavy rare earth element, a complex process that is so difficult to control and so on.
CN101316674A discloses a method for preparing a rare earth permanent magnet material. The method comprises the steps of disposing a powder of an oxyfluoride of a rare earth element on a surface of a magnet, treating the magnet at a temperature equal to or below the sintering temperature of the magnet so that the rare earth element is absorbed in the magnet, to thereby obtain a magnet with high performance by using a minimized amount of Tb or Dy. In this method, a powder of an oxyfluoride of a heavy rare earth element is diffused. The heavy rare earth element, on one hand, is detached from the oxyfluoride compound, on the other hand, needs to diffuse to the inside of the magnet. This needs a relatively long time for thermal insulation treatment, and may lead some problems. For example, a portion of the surface layer of the magnet becomes a Nd defect state and soft magnetic α-Fe or DyFe2 damages coercive force of the magnet. In addition, in this method, an oxyfluoride powder of heavy rare earth is dispersed in water or an organic solvent to obtain slurry, and then the slurry is disposed on the surface of the magnet. However, the slurry will be exfoliated easily during the operation due to the limited adhesive force between the slurry and the magnet, which results in an uneven absorption of the heavy rare earth element, thereby causing a poor consistency of performance of the magnet.
CN101331566A discloses an R—Fe—B rare earth sintered magnet and a method for producing the same. In this method, a sintered magnet and a container containing a heavy rare earth element are placed in the same processing chamber without contacting with each other; the heavy rare earth element is diffused from the surface of the magnet to the inside of the magnet by heating. In this method, non-contact diffusion and infiltration is adopted, so it can only rely on metal vapor. In this method, although diffusion can be even, the process is so difficult to control. If the temperature is too low, heavy rare earth vapor is difficult to diffuse from the surface of the magnet to the inside of the magnet, and the treatment time is significantly prolonged; when the temperature is too high, the formed heavy rare earth vapor of high concentration is much more than the vapor diffused to the inside of the magnet, so that a layer of heavy rare earth element is formed on the surface of the magnet, leading to a greatly reduced effect of grain boundary diffusion.
CN102568806A discloses a method for preparing rare-earth permanent magnets by the infiltration process, in which a fluoride of a heavy rare earth type element and metal calcium particles are placed at the bottom of a graphite box; and then slices of the magnet are placed; the fluoride of the heavy rare earth type element is reduced by the metal calcium; and then a heavy metal vapor is diffused to grain boundary phase of the magnet. This process is not described in detail, and can not be carried out easily. For example, details such as the fluoride of the heavy rare earth type element and the size of calcium particles which significantly affect the results of implementations are not mentioned. Moreover, the reduced heavy rare earth element is still diffused by a vapor process. Thus, there are deficiencies similar to those of CN101331566A.